Conventionally, laser diodes have been used as the light source in an optical disc drive, and laser beams emitted from the laser diodes have been known for their strong coherence. For this reason, in optical disc devices, a high-frequency superimposition method has been used as a method for reducing laser noise caused by the coherence between an outgoing beam from a laser diode and reflected light from an optical disc (see JP5-89465 A).
The high-frequency superimposition method is a method of, in a reproduction mode, producing a driving current in which a high-frequency current is superimposed on a DC current; and driving the laser diode based on the generated drive signal. Since the superimposition of such a high-frequency current can make spectrum of a laser beam turn into a multimode spectrum, the coherence of the laser beam can be reduced, and so laser noise due to reflected light from an optical disc can be reduced.
Furthermore, with respect to the high-frequency superimposition method, it has been known that the relationship between a light path length L between a light emitting point and an optical disc and the frequency F of high-frequency current at which the laser noise becomes lowest (hereinafter referred to as optimal frequency), is expressed in the below formula where c represents the velocity of light:
[Formula 1]f=c/(4×1)  (1)This formula (1) corresponds to pulse-driving the laser diode, in which the laser diode is turned off while the reflected light from an optical disc is re-entering the laser diode.